Osteoporosis is a disease characterized by low bone mineral density (BMD), the strongest predictor of fracture risk. Statistical genetic analyses in humans and animal models have clearly shown that multiple genes contribute to variation in bone traits and that BMD inheritance is gender and site-specific. Accordingly, gender specific BMD quantitative trait loci (QTL) have been identified in several chromosomes both in humans and animal models. However, no gender specific BMD QTL gene has been identified so far. Based on our recent findings that femurs from C57BL/6J (B6)-Castaneous (CAST) congenic mice carrying CAST alleles at D1mit115-D1mit152 (178-185 Mb, named BMD1-4 QTL) exhibit significantly greater peak BMD in the female but not male mice compared to corresponding sex-matched B6 mice, we have proposed the hypothesis that this region of chromosome 1 in CAST mice contains a gene that exerts gender-specific effect on BMD. By using additional subcongenic lines of B6.CAST-1 mice, we have narrowed down the size of the female specific BMD1-4 locus to 2.6 MB, a region small enough to proceed with the gene identification process. Our goal in this application is to investigate the mechanism by which the gene underlying BMD1-4 QTL regulates peak BMD in female mice and to screen the BMD1-4 locus to identify the candidate gene that imparts gender specificity. Towards this goal, we have proposed the following three specific aims. In specific aim 1, we will evaluate the skeletal phenotype of subcongenic lines of mice carrying BMD1-4 QTL gene and corresponding control mice at week 3 (prepubertal), 5 (pubertal), 8 (post pubertal) and 16 (adult). We will determine whether bone resortpion or formation is the main mechanism that contributes to the higher BMD in the female congenic mice with the anticipation that this information will assist in the selection of candidate genes for evaluation. In specific aim 2, we will evaluate the expression levels of the candidate genes underlying BMD1-4 locus in the bones of both male and female congenic and control B6 mice at a time when the phenotype starts to manifest. We will consider the gene/s that shows differential expression between male and female or between congenic and B6 mice as potential candidates. In specific aim 3, we will investigate the response of BMD1-4 gene to mechanical loading and determine if the response is gender-dependent. The successful identification of BMD1- 4 QTL gene that imparts gender specificity would lead to improved understanding of how skeletal growth is regulated and could provide novel drug targets for diagnosis and treatment of osteoporosis for each gender.